Irineu Batista and Rogério Mendes Institute of Fisheries and Marine Research Lisbon, Portugal
Bivalve molluscs Processing
The consumption of bivalve molluscs by humans dates back to the Late Archaic or Late Mesolithic periods. This is well documented by the shell middens found in many locations around the world. Bivalves continue to represent an important food item mainly for the population living near the rivers and seashore. In 1950, their production attained 1 034 000 tonnes (FAO, 2010) and in 2008 the total production (wild and farmed) was 13 841 000 tonnes (Figure 1). It is noteworthy that bivalves from aquaculture production represented about six times that caught in the wild.
Figure 1
Production of bivalve molluscs (farmed and wild) in 2008
Source: FAO, 2010.
Bivalve molluscs are usually marketed fresh as raw, unshelled or shucked refrigerated. They are also sold frozen, dried, canned, salted or in brine and marinated.
The shelf-life of bivalve molluscs is limited to the time they survive out of water.
This has led to different approaches to prolong their shelf-life such as reported for instance in the US Patent 5 165 361 (1992). In this patent a method is described to preserve bivalves in the live state in a closed container partially filled with water and replacing the air contained in the space with oxygen. The effectiveness of modified atmosphere packaging (MAP) for the preservation of fish and fish products has been recognized but only a few works were published on its application to bivalve molluscs.
Pastoriza et al. (2004) studied the stability of live mussels (Mytilus galloprovincialis) packaged under modified atmospheres. They obtained the highest survival in an atmosphere with high oxygen concentration (75% O2/ 2% N2), which allowed a shelf-life of 6 days when held at 2–3 °C. The shelf–life of control molluscs packaged in air did not exceed 3–4 days when stored under the same conditions. The application of MAP to preserve live clams (Ruditapes decussates) was also studied by
Gonçalves et al. (2009). Live clams stored both in air and packed in 70% O2/30%
N2 for 6 days at 6°C presented similar physiological conditions and health status.
However, a significant benefit of MAP storage was observed in the preservation of the characteristic sweet taste of clams.
The shelf-life of post-mortem bivalves is very short because of the high water activity, neutral pH, high amino acid content and also the presence of psychrotolerant spoilage bacteria. On the other hand, the spoilage mechanism associated with bivalves is different from that of crustaceans and finfish because of the presence of significant levels of carbohydrate, which leads to saccharolytic activities and the accumulation of organic acids. Bivalve molluscs as water-filtering organisms accumulate microorganisms, which are closely related to the environmental conditions, microbiological quality of the water where they live and other physicochemical characteristics of the habitats. Pathogen rich microflora may be also present in bivalves, particularly on those inhabiting estuaries, which makes them more susceptible to the faecal contamination and environmental pollution of the surrounding waters. In fact bivalves are highly featured in statistics of food-borne diseases.
The effect of ozonation in aqueous solution on the shelf-life of shucked, vacuum packaged mussels, stored under refrigeration was studied by Manousaridis et al. (2005).
Ozonation reduced bacterial populations and on the basis of sensory analyses, a shelf-life of 12 days was obtained for vacuum packaged mussels ozonated for 90 min as compared with a shelf-life of 9 days for non-ozonated vacuum packaged mussels.
In order to increase the shelf-life of mussels a combination of MAP technology and refrigeration was reported by Goulas (2008). The best results were achieved with the mixture 60% CO2/20% N2/20% O2, which kept the mussels acceptable up to ca.
10–11 days based on the odour scores. In a similar study, Caglak et al. (2008) studied the microbiological, chemical and sensory changes occurring in mussels stored aerobically, under vacuum and three modified atmospheres (50% CO2/50% N2, 80% CO2/20%
N2, 65% CO2/35% N2). According to these authors the gas mixture richest in CO2 was the most effective for mussel preservation, which were acceptable for 8 days of storage.
Scallops are also valuable bivalve molluscs where MAP has been applied to increase their shelf-life. This technology was used by Kimura et al. (2000) to preserve the scallop adductor muscle stored at 5°C in an atmosphere of 100% O2, 80%
O2/20%CO2, 60% O2/40% CO2, and air. The best results were obtained with 100%
O2 atmosphere, which allowed a prolongation for nearly two days in shelf-life of the scallop adductor muscle. Simpson et al. (2007) studied the optimal conditions for packaging scallops (Argopecten purpuratus) in modified atmosphere system. According to the mathematical model developed in this study the optimal conditions for scallop storage were a 60% CO2/10% O2/30% N2 gas mixture and a headspace:food ratio of 2:1. With these conditions, a simulated shelf-life of 21 days was obtained.
The demand for safe foods, additive free, fresh tasting and with extended shelf-life has led also to the utilization of high pressure (HP) treatment of bivalves, particularly oysters. This treatment has the potential to improve microbial quality without compromising sensory and nutritional quality (Farkas and Hoover, 2000).
Furthermore, the application of HP kills the oyster and facilitates the opening by hand or may even be used to induce shucking. As reported by Lopez-Caballero et al. (2000) HP treated oysters preserved their raw appearance, were slightly more voluminous and juicier and the flavour was virtually unchanged. HP treatment of oysters (200–400 MPa/7°C/10 min) reduced the number of all targeted microorganisms. The appearance of the oyster meat was better when pressurization (400 MPa) was carried out under chilled conditions (7 °C) rather than at higher temperatures (20°C and 37°C). Calik et al. (2002) showed that Vibrio parahaemolyticus (Vp) numbers were reduced by HP treatment in both pure culture and whole Pacific oysters. Optimum
conditions for reducing Vp in pure culture and whole oyster to non-detectable levels were achieved at 345 MPa for 30 and 90 s, respectively.
In a previous work He et al. (2002) also observed a reduction of the initial microbial load by 2 to 3 logs in HP treated Pacific oysters. The reduced bacterial counts remained low through the storage period at < 4 °C. The pH of HP treated oysters decreased slightly from 6.3 to 5.8 during storage while the hand shucked oysters (control) dropped to 4.1, this sharp decrease being a clear indicator of bivalve spoilage (Jay, 1996). HP treated oysters received higher quality scores than controls during the storage trial.
In the study by Linton et al. (2003) it is concluded that pressure treatment of mussels, scallops and oysters at 300, 400, 500 and 600 MPa for 2 min at 20 °C readily inactivated psychrotrophic bacteria, coliforms and Pseudomonads. The range of bacteria present in the products decreased after pressure treatment mainly because of inactivation of Gram negative bacteria. This led to an increase of proportion of Gram positive species (Bacillus, Acinetobacter/Moraxella and lactic acid bacteria).
Cruz-Romero et al. (2008a) studied the changes in microbiological and physicochemical quality of oysters HP treated at 260 – 600 MPa for 5 min and stored at 2 °C on ice for 31 days. This study confirmed that the HP processing of oysters can inactivate microorganisms and delay microbial growth in chilled storage, but also showed that it affects their quality attributes. In another study Cruz-Romero et al. (2008b) followed the microbiological and biochemical changes in high pressure treated oysters stored aerobically on ice, in vacuum packaging and under MAP (40% CO2/60% N2). The use of MAP was shown to be effective in extending the shelf-life of HP treated oysters and according to the authors has great potential for preserving HP treated oysters.
The potential of HP processing to reduce viral contamination in mussels and oysters was also demonstrated by Murchie et al. (2007). Bovine enterovirus, structurally similar to hepatitis A virus, was more pressure resistant than feline calicivirus, a surrogate for norovirus. Both viruses were more pressure resistant when treated in “naturally” contaminated mussels and oysters, compared with seawater and culture medium. The results obtained suggested that relatively mild HP treatments (approximately 260 MPa) currently used for commercial processing of oysters, may be insufficient to ensure the safety of shellfish for human consumption, particularly in relation to human pathogenic viruses (Figure 2). In the work by Kingsley et al.
(2007) it is demonstrated that a marine norovirus (strain MNV-1) can be inactivated by high pressure. A 5 min, 450 MPa treatment was sufficient to inactivate 6.85 log PFU of MNV-1 in virus stock in Dulbecco’s modified Eagle medium. The inactivation of MNV-1 directly within oyster tissue was also achieved, a 5 min 400 MPa treatment at 5°C to inactivate 4.05 log PFU was sufficient. Taking into account that cooking may not be enough to avoid shellfish borne virus transmission (McDonnel et al., 1997) HP treatment may therefore be useful for reducing infectious virus in bivalves prior to cooking.
Oyster shucking by HP processing is at present and for the last five years a well known process with commercial success by several North American and European companies (Raghubeer, 2007). Using HP patented technology, no-shell shucked oyster and a fully detached and ready-to-serve frozen half shell oyster are awarded products from Gold Band Oysters and good examples of the exploitation of this process and of the technological advances in this field. For this specific purpose, shucking, one considerable downside of HP processing is the capital investment. An affordable cost may however be offered by other technologies under investigation. For example, with the joint utilization of oyster positioning and imaging technologies (So and Wheaton, 2002) the precise application of a laser to the shell immediately above the adductor muscle is a promising technology. According to Martin and Hall (2006), the exact
application of heat precisely above the muscle scar results on a very clean release of the adductor muscle while keeping the oyster raw.
Figure 2
effect of high pressure treatments on the infectivity of bovine enterovirus in culture medium
Note: effect of HP treatments (150-550 MPa for 5 min at 20 °C) on the infectivity of bovine enterovirus (BeV) in culture medium (■), seawater (●), mussels (□) and oysters (○). Average tissue culture infectious dose for 50% (TCiD50) obtained from three independent trials. error bars are standard error of the mean. aBeV was detected in two out of three trials (value shown calculated from results of two trials). bBeV was detected in one out of three trials (TCiD50 ≤ 1.0) (Murchie et al., 2007; reproduced with permission of elsevier Limited).
High pressure processing was also applied to thawing scallops (Pecten irradians).
Optimal results were obtained at 150 MPa, and achieved a significantly reduced drip loss (31 percent) when compared with thawing under atmospheric pressure (Flick Jr., 2003).
The effect of HP processing on the quality of scallop (Aequipecten irradians) adductor muscle was also studied by Pérez-Won et al. (2005). This work has shown that HP processing induced a size reduction of the honeycomb structure of myofibres giving a more compact appearance to the structure. This HP treatment also reduced initial load in total plate count of microorganisms to 10 cfu/g. The colour and compressibility of HP treated scallops were enhanced but loss of hardness was observed.
The restructuring process at low temperatures is a technological alternative for the upgrading of underutilized resources, which have an unappealing aspect or small size. This process was applied by Suklim (1998) to upgrade calico scallops (Argopecten gibbys) by using alginate and MTGase (Microbial Transglutaminase) at 1 percent level as cold-set binders with different setting times. At the setting temperature of 5°C, restructured scallops bound with alginate presented the greatest binding strength at 2 hrs setting, while those bound with MTGase required 24 hours to reach the maximum binding strength. However, the products obtained with alginate had lower binding strength values, which may result in a decrease in consumer acceptability. Beltrán-Lugo et al. (2005) also made the restructuring of small or broken pieces of the adductor muscle of the lions-paw scallop (Nodipecten subnodosus) and the catarina scallop (Argopecten ventricosus) to obtain uniform and commercial size scallop meat. Two cold-set binding systems – caseinate-transglutaminase (CT) and fibrinogen-thrombin (FT) –were used.
The results obtained led to concluding that lions-paw and catarina scallops can be successfully restructured by CT and FT systems (Figure 3). They also indicated that, not only the restructuring system, but the species have influence on characteristics of restructured scallop meat. The end colour of the FT system was noticeable in the
adductor muscle from lions-paw scallops. A larger increase in most texture parameters was produced by the CT system than was produced by the FT system.
Figure 3
shear tests and light microscopy of scallop meats
Note: (A) Warner-Bratzler shear test values of raw materials and restructured meats of lions-paw scallops and catarina scallops using two cold-set binding systems. Different letters within species indicate significant differences (P<0.05) between treatments. Bars represent standard deviation (n = 10). (B) Light microscopy images (40x) contrasted with Masson’s trichrome stain of polymerized matrices and restructured meats of the two scallop species using CT and FT systems. A = CT matrix; B = FT matrix; binder-adductor muscle interface for lion-paw scallop meats restructured wit CT (= C) and FT (= e). Binder-adductor muscle interface for catarina scallop meats restructured wit CT (= D) and FT (= F). Muscle fibers (MF) and polymerized proteins of matrices (PPM) appear as pink to red colour. interstitial materials appear white. CT = restructured with casein-transglutaminase; FT = restructured with fibrinogen-thrombin; rM = raw material. (Beltrán-Lugo et al., 2005; reproduced with permission of John Wiley and Sons).
cePhaloPod Processing
Cephalopods landings increased from around 600 thousand tonnes in 1950 to more than 4.3 million tonnes in 2008 (FAO, 2010). This enormous increase of cephalopod landings was previously foreseen by Caddy and Rodhouse (1998) who considered that
“cephalopod fisheries are among the few still with some local potential for expansion”.
Figure 4 shows the evolution of cephalopod landings and the percentage of different groups of commercialized cephalopods.
Cephalopods are fishery products very much appreciated in the Mediterranean and Asia. They deteriorate more rapidly than fish and under refrigeration have a relatively short shelf-life. Autolysis of the cephalopod muscle is particularly intense because of the high level of proteolytic activity produced by their highly active metabolism.
As a consequence, the products resulting from the autolytic activity favour rapid microbial growth. Thus, alternative technologies to refrigeration on ice have been tried to extend the shelf-life of cephalopods. The application of modified atmospheres is one of those technologies, having been used by Ruiz-Capillas et al. (2002) to preserve pota (Todaropsis eblanae) and white octopus (Eledone cirrhosa). The results reported by these authors indicated that a controlled atmosphere with 60% CO2/15% O2/25%
N2 together with refrigeration at 1°C increased the shelf-life of both species by at least 54 percent.
Note: (A) Total catches of cephalopods from 1950 – 2009; (B) Breakdown of production by groups of species in 2008 Source: FAO, 2010.
A combination of vacuum-packaging and oregano essential oil (0.4% v/v) was also applied to preserve octopus (Octopus vulgaris) during storage at 4°C (Atrea et al., 2009). Based primarily on sensory evaluation (odour), the use of those conditions allowed extending the shelf-life of fresh octopus by approximately 20 days.
An important characteristic of octopus is its toughness, which makes it nearly inedible if it is cooked without previous tenderization. This property of octopus muscle led Katsanidis (2004) to study the effects of tumbling time, NaCl concentration, boiling time, and acetic acid levels on the tenderness of fresh octopus (Eledone moschata). The author concluded that prolonged tumbling and heating of octopus muscle resulted in decreased toughness. Addition of NaCl during tumbling did not affect toughness consistently. On the other hand, acetic acid at levels of 0.1 percent and 0.2 percent significantly reduced toughness of octopus muscle. In a similar study, Katsanidis and Agrafioti (2009) evaluated the effect of using acetic, lactic and citric acids on the tenderization of octopus (Octopus vulgaris). The addition of these acids at 0.05 and 0.1M levels resulted in significant tenderization compared with the untreated control. Although no differences in the tenderizing effect within acids was observed, their use shortened the heat processing time of octopus almost by half.
Other approaches have been tried to softening cephalopods, mainly dried squid.
This is a popular seafood product in several Far East countries, which can be cooked directly with or without prior softening. This process may be performed by various rehydration processes but immersion of dried squid in alkaline solution has become a widely used method. Kugino et al. (1993) studied the differences between raw squid and softened dried squid under various conditions. Electron microscopy showed water permeation throughout the muscle fibrils and fibres, while there was almost no permeation of water inside the individual fibrils. In order to investigate the effect of some processing parameters of alkaline treatments on the physicochemical properties of dried squid Benjakul et al. (2000) used different NaOH or Na2CO3 solutions for soaking. They concluded that dried squid soaked in 0.15 mol.kg-1 NaCO3 with a squid/alkaline solution ratio of 1:10 (w/v) for 20 h was the most acceptable in terms of both appearance and textural properties. In another study on softening dried squid prepared at 4 and 40°C performed by Konishi et al. (2003) it was concluded that a significantly higher wet weight was observed when processing was done at 4°C. The protein pattern obtained by SDS-PAGE of the 4°C dried squid was almost the same as that of raw squid.
High pressure treatment is another interesting alternative for preserving cephalopods. In one of the first works (Matser et al., 2000) on the application of HP to octopus (Octopus vulgaris) at 0°C and 5 min pressure holding time, it was concluded
Figure 4
cephalopod production data
that octopus retained a raw appearance till 400–800 MPa. In the work by Hurtado et al.
(2001) the application of HP (400 MPa) continuously or in pulsed form at 7 and 40°C to octopus is reported. A reduction of microbial flora (total viable count and lactic bacteria) after pressurization and during chilling storage was recorded. This reduction was more significant in the lot pressurized by step-pulse. A lower level of nitrogenous compounds and a decrease of the autolytic activity were obtained in the pressurized octopus in comparison with control samples. The shelf-life of the pressurized octopus was 43 days longer than unpressurized.
The application of HP treatment to squid (Todaropsis eblanae) mantles was studied by Paarup et al. (2002). These authors evaluated the changes occurring in vacuum packed pressurised squid mantle during refrigerated storage (4°C). Squid mantles were pressurised in the range between 150 to 400 MPa for 15 min at ambient temperature.
The sensory analysis showed that the higher the pressurisation the longer the shelf-life.
Microbial counts conducted after one day of storage showed a reduction of bacterial loads in all pressurised lots, reaching levels below the detection limit in the lots treated with 200–400 MPa.
In a recent paper (Gou et al., 2010) the effect of HP processing on the quality of squid (Todarodes pacificus) during refrigerated storage is described. This work is particularly focused on the effect of HP on the reduction of unpleasant off odours.
Thus, the influence of HP treatment on the inhibition of trimethylamine-N-oxide demethylase (TMAOase) activity and microbial growth in squid treated at 300 MPa for 20 min was investigated. TMAOase activity and the production of dimethylamine in raw squid were significantly reduced after HP treatment. Similarly, the number of total aerobic bacteria was also reduced by 1.26 log units and HP treated squid products presented a lower production of trimethylamine.
Concerning changes during cooking, early studies on texture changes in cooked squid muscle using scanning electron microscopy date back to the 1970s (Otwell and Hamann, 1979). Thermal alterations of muscle fibres appeared as a loss of myofibril distinction first evident at 50°C. Increasing temperature of muscle fibres caused, in order, coagulation of sarcoplasmic proteins, disintegration of the sarcoplasm, and continuous fibre shrinkage and dehydration. Later on Otwell and Giddings (1980) reported that squid muscle heated at 100°C showed gross distortions of all mantle tissues. Mieko et al. (2000) also studied the textural changes occurring in three cooked squid species (oval squid, Japanese common squid and arrow squid). These authors concluded that the speed of squid muscle becoming tough and then tender depended on the squid species. The fastest tenderization was observed in arrow squid followed by the Japanese common squid and the slowest softening was recorded in the oval
Concerning changes during cooking, early studies on texture changes in cooked squid muscle using scanning electron microscopy date back to the 1970s (Otwell and Hamann, 1979). Thermal alterations of muscle fibres appeared as a loss of myofibril distinction first evident at 50°C. Increasing temperature of muscle fibres caused, in order, coagulation of sarcoplasmic proteins, disintegration of the sarcoplasm, and continuous fibre shrinkage and dehydration. Later on Otwell and Giddings (1980) reported that squid muscle heated at 100°C showed gross distortions of all mantle tissues. Mieko et al. (2000) also studied the textural changes occurring in three cooked squid species (oval squid, Japanese common squid and arrow squid). These authors concluded that the speed of squid muscle becoming tough and then tender depended on the squid species. The fastest tenderization was observed in arrow squid followed by the Japanese common squid and the slowest softening was recorded in the oval